1. Field of the invention
The present invention relates to a valve driving mechanism for internal combustion engine, more specifically to a cam shaft control mechanism therefor.
2. Description of the prior art
There has been known an engine provided with a double over head cam (DOHC) type valve driving mechanism for driving intake and exhaust valves disposed over a cylinder head by means of a pair of cam shafts also disposed over the cylinder head. For instance, Japanese Utility Model Public Disclosure No. 61-9501, laid open to the public in 1986, discloses a valve driving mechanism in which one of over head cam shafts is provided with a driving sprocket or driving pulley in case of timing belt and the other of the cam shafts is connected with the one of the cam shafts through a gear mechanism so that driving force is transmitted from the one to the other of the came shafts through the gear mechanism.
This type of valve driving mechanism is advantageous in that a compact mechanism can be obtained.
In the valve driving mechanism, there has been proposed a valve timing varying system which is effected to vary opening and closing timing and thus, an overlap period of valve opening in accordance with engine operating condition so as to improve engine output property.
Japanese Patent Public Disclosure (KOKAI) No. 60-240809, laid open to the public on Nov. 29, 1985, U.S. Pat. No. 4,535,731 issued on Aug. 20, 1985 and U.S. Pat. No. 4,674,452 disclose various valve timing varying systems of the valve driving mechanism.
In varying the valve timing, there has been proposed providing a rotation phase changing device for changing a rotation phase between intake and exhaust cam shafts wherein the rotation phase changing device is constituted by a mechanism as utilizing helical splines arranged between the driving pulley connected with a crank shaft and the cam shaft.
It should however be noted that in the valve driving mechanism in which a driving force is transmitted from one to the other of the cam shafts through a gear mechanism disposed therebetween, the rotation phase of the cam shafts relative to each other cannot be changed and therefore, the overlap period of the valve opening cannot be controlled.
Other valve timing varying systems tends to be complicated in mechanism.